ASTM B497-00 pdf free.Standard Guide for Measuring Voltage Drop on Closed Arcing Contacts.
This guide covers the factors to be controlled, precau-tions and documentation necessary to measure and report thevoltage drop across closed current-carrying contacts. Thevoltage drop is an indication of the efficiency of the contactinterfaces in carrying a specified current. This efficiency can beadversely effected by any insulating areas within the contactinterface. Circuits which involve substantial current and lowindependence can be influenced by this contact property.If a selfcontained instrument is used,the followingprecautions should be observed:The voltmeter leads should be connected as shown inFig. 1 as close to the test contacts as possible.The leads should be as short as possible, or calibratedwith the meter，and shielded to reduce the effect of straypickup voltages.The voltmeter input impedance should be 1000 orless.This will load the circuit sufficiently to reduce the effectsof stray-voltage pickup in the instrument leads.The inputimpedance of high-impedance meter can be reduced by placinga 1000-Q resistor, B, across its input terminals as shown in Fig.1.Before measurements are made,the voltmeter leadsshould be shorted together at point C. With the voltmeter leadsshorted in this position,the test current should be caused toflow in the circuit. If the voltmeter deflects， it may be causedby induction from the load circuit to the internal components ofthe meter. This may be minimized by orienting the meter，orrelocating the meter (that is, moving it away from the circuit)until minimum meter deflection is observed.If necessary, placeconductive shielding between the meter and the circuit.  To determine if the ground-loop impedance, illustratedby resistance A in Fig. 2 is detrimental to the meter short theinstrument leads to each other at point C. If the meter gives adeflection with the leads so connected when contact currentflows，the following steps must be taken to remove thedisturbance:Eliminate the ground loop formed by the connec-tions of the electronic voltmeter to the power line. This loop isshown by the broken line in Fig. 2.It can be completed throughinterwinding capacitance of the instrument power transformer.This capacitance effect can be reduced by placing an additionalisolation transformer between the measuring instrument andthe source of instrument power (Fig. 3).An optional technique that can be used to eliminatethe effect of the ground loop, is to use a differential voltmeter.A differential voltmeter is an analog device that continuouslymeasures the difference between two potential levels withrespect to ground as a reference point.ASTM B497 pdf download.